Process for curing thermoset resins using phenyl esters of carboxylic acids as latent catalysts

ABSTRACT

Latent catalysts comprising the phenyl esters of certain carboxylic acids are disclosed for use in the cure thermosetting resins curable by an acid catalyst.

The present invention involves the novel use of certain phenyl esters ofcarboxylic acids as catalysts in the cure of thermosetting resins. Thesecatalysts are latent, being catalytically inactive at lowertemperatures, but become catalytically active at temperaturesencountered during the curing of the resin.

Thermosetting compositions have been known for many years and have beenemployed in many applications because of such advantageous properties aslight weight, high heat resistance and excellent dimensional stability.The traditional method for processing thermoset molding compoundsinvolves blending with fillers, pigments and other additives, followedby compounding and granulating. The granulated compositions are thenfabricated by any of the well known methods such as compression,transfer, or injection molding. This multi-step process is cumbersomeand energy intensive. Furthermore, the compounding step often causesconsiderable attrition of the reinforcing fiber and thus the reinforcingaction of the fiber is not effeciently utilized. As a consequence,ultimate high impact resistance is often not obtained.

Recently, there have been devised thermoset fabrication methods in whichliquid thermosetable compositions are injected directly into a moldwhere cure takes place resulting in the formation of a fabricated part.Depending on the injection process and the nature of the compositions,these methods are known as liquid injection molding (LIM), reactioninjection molding (RIM), or resin transfer molding (RTM).

Another process for manufacturing high strength thermoset composites isthe sheet-molding compound (SMC) method. In this process, a liquidthermosetting resin, such as an unsaturated polyester, reinforcing fiberand other additives are mixed under low shear conditions. The resultingviscous mixture is partially cured to non-tacky sheets. Final cure tofinished parts is then carried out in a mold. The commercialapplications of this method have been generally limited to unsaturatedpolyesters which, unless they are specially treated, exhibit poorflammability resistance. In addition, careful formulation is required toobtain good processability and smooth surfaces.

These processes are rapid and adaptable to high speed productionrequirements, and since the curing reaction is generally exothermic,these processes are less energy intensive than the traditional methodsfor processing thermosets. Furthermore, because these relatively newthermoset fabrication methods are low pressure processes, there isrequired considerably lower clamping forces than those required for theinjection molding of engineering plastics, and thus lower capital andoperating expenditures are required.

It has recently been found that certain resins are particularlyadaptable for use in injection molding processes because of their lowviscosity and their having a low amount of unbound water. These resinsare disclosed in U.S. Patent Applications Ser. No. 340,853 and 340,855filed concurrently herewith by Brode and Chow, and Brode, Chow and Hale,respectively. These resins are high-ortho resoles containing hemiformalgroups. They are of low viscosity, are essentially free of unbound waterand can be cured to phenol-formaldehyde thermoset shapes and when mixedwith reinforcing material to composite shapes.

Another class of thermosetting compositions found suitable in injectionmolding processes are hemiformals of phenol and methylolated phenol andsolutions of these hemiformals as disclosed in U.S. Patent ApplicationsSer. No. 340,719 filed by Covitz, Brode and Chow, Ser. Nos. 340,790 and340,720 filed by Brode and Chow and Ser. No. 340,695 filed by Brode,Chow and Hale, all on Jan. 19, 1982. These compositions are hemiformalsformed by the reaction of formaldehyde with the phenol hydroxy andmethylol groups of phenol. These compositions are stable and are of lowviscosity and can be used to form phenol-formaldehyde resin. They canalso be mixed with a co-reactive polymer to form solutions useful inliquid injection molding and are curable to solid shapes.

Generally, the catalyst used for curing the above phenol-formaldehyderesins and other thermosetting compositions has been acids or bases.Suitable acid catalysts that have been used include sulfuric acid,phenyl sulfonic acid, phosphonic acid, oxalic acid, ferric chloride, andtoluene sulfonic acid. These acids have been used successfully withthermosetting resins, such as those disclosed above. However, theseacids have significant catalytic activity, even at ambient temperatures(20°-30° C.). Therefore, a resin that contains an acid catalyst veryquickly begins to gel and form a solid resinous product after thecatalyst has been added. Therefore, when used in liquid injectionmolding processes the acid catalyst is typically mixed with the resinshortly before injection into the mold. A less expensive and moreconvenient method would be to mix the catalyst with the resin and thenstore it until it is needed. However, because of the activity ofconventional acid catalysts at storage temperatures, an attempt to storea resin containing an acid catalyst would risk gelling andsolidification of the resin in the storage areas and process lines. Itwould, therefore, be highly desirable to have available a catalyst thatis latent, namely one that exhibits little or no activity at ambientconditions in which the resin is stored, but becomes catalyticallyactive at the cure conditions found in a mold. Then the catalyst couldbe added to the resin and the resin stored until used without danger ofthe resin gelling.

It has now been found that certain phenyl esters of carboxylic acids canbe used as latent catalysts in the curing of thermosetting resins thatare curable by an acid catalyst.

These catalysts when mixed with thermosetting resins show little orinsignificant activity at storage temperatures, allowing storage of thecatalyst-containing resin. However, when exposed to curing temperatures,they become very catalytically active and facilitate rapid curing of theresin.

The catalysts useful in the method of the invention are phenyl esters ofcarboxylic acids that hydrolyze to form acids having a pK₂ of about 2 orless when exposed to water and temperatures greater than about 100° C.These compounds when mixed with a resin at storage temperatures, about60° C. or less, are neutral or only weakly acidic. Therefore, theircatalytic activity is very small at storage conditions. However, whenexposed to curing temperatures, about 100° to 200° C., they produce insitu strong acids that catalyze the curing of the resin.

At storage conditions the latent catalyst exists as the unhydrolyzedphenyl ester which is neutral or only weakly acidic. This is due to thetemperature being too low to activate a hydrolysis reaction and/or theabsence of water in the resin. When the resin is heated to curetemperatures, it undergoes a heat activated condensation reaction inwhich water is released. The phenyl ester latent catalyst hydrolyzes atthe cure temperature and in the presence of the water from thecondensation reaction, and other water that may be present, to form acarboxylic acid that acts as an acid catalyst. This process isaccelerated as the carboxylic acid catalyzes the condensation reaction,producing more water. Thus, a highly active acid catalyst is producedwhich effectuates a rapid cure of the resin. The condensation reaction,whereby the resin is cured, is further accelerated by the removal ofwater from the system. The rate of cure is inversely proportional to theconcentration of water, so removal of water by hydrolysis of the phenylester increases the rate.

The preferred phenyl esters of carboxylic acid include phenyltrifluoroacetate and phenyl hydrogen maleate. Phenyl trifluoroacetate isneutral and phenyl hydrogen maleate is a weak acid (pK_(a) greater than3). Both yield strongly acidic products upon hydrolysis, namely,trifluoroacetic acid (pK_(a) =0.27), and maleic acid (pK_(a) =1.94),respectively.

The phenyl esters may be introduced as such or made in situ by reactingan anhydride of a suitable carboxylic acid, such as maleic anhydride,with phenol to form the phenyl ester.

Other suitable phenyl esters include the phenyl esters of the followingacids: 4-chloro-o-phthalic acid (pK_(a) =1.6), dibromosuccinic acid(pK_(a) =1.5), 2,6-dihydroxybenzoic acid (pK_(a) =1.3), andbut-2-yne-1,4-dicarboxylic acid (pK_(a) =1.75).

Esters of polymers that have acid groups incorporated into theirstructure can also be used, providing they have the above recitedhydrolysis properties at elevated temperatures. Examples includecopolymers of vinyl alkyl ether-maleic anhydride and styrene-maleicanhydride. These polymeric acid-esters when used in sufficientquantities can also serve to modify polymer properties.

The phenyl ester latent catalyst is introduced in an amount to give aconcentration of about 0.2 to about 10 weight percent, preferably about0.5 to about 5 weight percent, based on the weight of the catalyst freeresin.

Resins suitable for use in the method of the invention are thermosettingresins that are curable using an acid catalyst. These includephenol-formaldehyde, urea-formaldehyde, and melamine-formaldehyderesins. Preferably the resins used in the method of the invention areusable in sheet molding compounds or in injection molding processes. Ifused in sheet molding compound processes, the resin used in the methodof invention should have a viscosity less than about 500,000 centipoise(Brookfield) at 25° C.

When used in liquid injection molding processes, the resin should have aviscosity less than about 10,000 centipoise (Brookfield) at 25° C.Resins suitable for use in the method of the invention include thehemiformals of phenol and methylolated phenol and solutions of thesewith a co-reactive polymer as disclosed in the above cited UnitedStates, Applications, Ser. Nos. 340,719, 340,790, and 340,720. Theseresins are described as having any one of the formulas below, ##STR1##wherein R is any substituent typically employed in conjunction with aphenolic structure, n is a positive number of at least 1, preferablyabout 1 to about 5, most preferably about 1.2 to about 2.5. b is 1 toabout 5, c is 1 to about 3, and d is 0 to about 2, x is 0 to 3, the sumof c and d is at least 1 and no greater than 3 and the sum of c, d, andx is at least 1 but no greater than 5, where x=0 or at least 50 molepercent of the hemiformal, and with respect to the R substituent, atleast 2 of the ortho- and para- positions are free in relation to the--OH and --O(CH₂ O)_(n) H groups. With respect to R, it is preferably amonovalent radical which includes alkyl of from 1 18 carbon atoms,cycloalkyl from 5 to 8 carbon atoms, aryl containing from 1 to 3aromatic rings, aralkyl, alkaryl, alkoxy containing from 1 to 18 carbonatoms, aroxy containing 1 to 3 aromatic nuclei, halide such as chloride,bromide, fluoride, and iodide; alkyl sulphido having from 1 to 18 carbonatoms, aryl sulphido having from 1 to 3 aromatic nuclei, and the like,as well as, a radical derived from an oil such as linseed oil or tungoil. Examples of co-reactive polymers are phenol-formaldehyde resoles,phenol-formaldehyde novolacs, aromatic polyesters, aromaticpolycarbonates, unsaturated polyesters, poly(aryl-ethers),urea-formaldehyde resin, and melamine-formaldehyde resins.

Also included in the resins suitable for use in the invention are thephenolic resins disclosed in U.S. Pat. No. 3,485,797, issued to Robinson Dec. 23, 1969. These are phenol formaldehyde resins having thegeneral formula ##STR2## wherein R is hydrogen, hydrocarbon radical,oxyhydrocarbon radical or halogen, meta to the hydroxyl group of thephenol; m and n are numbers the sum of which is at least two and theratio of m to n is greater than one; and A is a hydrogen or a methylolgroup, the molar ratio of said methylol group to hydrogen being at leastone.

A preferred resin for the method of the invention is disclosed in theabove cited application, U.S. Ser. No. 340,853. These resins arehigh-ortho phenol-formaldehyde resole polymers containing hemiformalgroups and having the general formula ##STR3## wherein a is from 0 to 3,b is 0 to 1, the sum of a and b does not exceed 3, the sum of c and d isfrom 2 to about 20, the mole fraction d/(c+d) is 0.4 to 0.9, preferably0.6 to 0.8, R is --CH₂ O(CH₂ O)_(e) H, e is 0 to about 5, and X is amonovalent radical, wherein for at least one of the R or O(CH₂ O)_(e) Hgroups e is at least 1 and wherein at least 50 mole percent of the##STR4## X is any substituent typically employed in conjunction with aphenolic structure. With respect to X, it is preferably a monovalentradical which includes alkyl of from about 1 to about 18 carbon atoms,cycloalkyl from 5 to 8 carbon atoms, aryl containing from 1 to about 3aromatic rings, aralkyl, alkaryl, alkoxy containing from 1 to about 18carbon atoms, aroxy containing from 1 to about 18 carbon atoms, aroxycontaining 1 to 3 aromatic nuclei, halide such as choride, bromide,fluoride, and iodide, alkyl sulphides having from 1 to about 18 carbonatoms, aryl sulphides having from 1 to about 3 aromatic nuclei, and thelike. These resins are essentially free of unbound water and volatileorganic compounds and have a low viscosity, as low as 1,000 centipoise(Brookfield) at 25° C. By essentially free of unbound water is meantthat the resin contains less than about 5 weight percent, preferablyless than about 2 weight percent, of unbound water and volatile organiccompounds, based on the total weight of the resin. Concentrations lessthan 1 weight percent are achievable. By unbound water is meant thatwater present as an impurity and is distinguished from the waterproduced from the condensation reaction. Volatile organic substances arethose that volatilize to form a gas when the resin is exposed toelevated temperatures, about 100° C. These include formaldehyde notincorporated in methylol groups, hemiformal groups on methylol linkagesof the resin. Also includes are the solvents typically used to reducethe viscosity of a resin, such as alcohol or aromatic hydrocarbons. Alsoincluded are such substances such as methanol that may be introduced ascontaminants in the formaldehyde or phenol used in manufacture of theresin.

The amount of water that can be tolerated in the resin used in themethod of the invention depends on the susceptibility of the phenylester catalyst towards hydrolyzation. For example, phenyl hydrogenmaleate at temperatures below about 60° C. will hydrolyzeinsignificantly, even in the presence of a significant amount of waterin the resin. However, at elevated temperatures and when in the presenceof water, phenyl hydrogen maleate will dissociate to form maleic acid.Therefore, when phenyl hydrogen maleate or a phenyl ester having similarhydration properties is used, resins having significant amounts of watermay be used. Resins containing up to about 5 weight percent water havebeen found satisfactory.

On the other hand, phenyl trifluoroacetate hydrolyzes and forms acidsquite readily at low temperatures. Therefore, when using a phenyl esterthat hydrolyzes readily, such as phenyl trifluoroacetate, as a latentcatalyst the resin should contain less than about 1 weight percent,based on the weight of the resin, of unbound water. Such a suitableresin is that disclosed in the above cited United States Ser. No.340,853, wherein the resin is made under azeotropic distillationconditions to remove residual water.

The resin containing the latent catalyst can be cured using knownmethods in the art. The preferred curing method is any one of the liquidinjection molding processes wherein the resin is injected into a mold,and cured by heating the mold. The catalyst may be premixed with theresin and stored before the curing. The catalyst containing resin isstable at storage temperatures with stability decreasing as thetemperature rises. Storage temperatures lower than 60° C. are suitablealthough storage temperatures lower than 30° C. are preferred.

The catalyst containing resins can be cured at temperatures betweenabout 100° C. and about 200° C., preferably between 120° C. and about180° C. The optimum curing temperature for the most rapid cure ratedepends in large part on the kinetic properties of the particular latentcatalyst used. For phenyl hydrogen maleate, the optimum curingtemperature is about 150° C. to 160° C. For trifluoroacetate, theoptimum curing temperature is about 140° C.

The rate of the cure can be regulated by the cure temperature and theconcentration of the catalyst.

Optionally, resins containing the latent catalyst may be blended with areinforcing material such as glass fiber, graphite fiber, carbon fiber,wollastonite, cellulousic fibers such as wood fiber and the like,organic fibers such as aromatic polyamide fibers, and mica. Thepreferred reinforcing materials are glass fiber, graphite fiber, carbonfiber, and aromatic polyamide fiber. These fibers may be in any formcommon in the art, such as chopped fiber, mat, and woven cloth.

When used in sheet molding compound processes, the catalyst-containingresins can be blended with a chopped fiber and cured to a B-stage resinand then later to finished composite shapes.

In liquid injection molding processes, the fiber may be placed in anysuitable form into the mold before injection of the catalyst-containingresin.

The following examples demonstrate the invention and are not intended tolimit the invention in any way.

EXAMPLE 1

Evaluations of phenyl hydrogen maleate and phenyl trifluoroacetate aslatent catalysts were made by obtaining differential scanningcalorimetric data (DSC) and the viscosity vs. time relationships.

DSC is a method for determining the catalytic activity of a catalyst.The cure of phenol-formaldehyde resins is exothermic; therefore, thetemperature of the exothermic peak of a DSC spectrum is a measure of thetemperature where the most rapid cure rate occurs and the activity ofthe catalyst during cure.

The relationship of viscosity vs. time is a measure of stability at agiven temperature. The stability can be measured by the time it takesthe viscosity to double. An increase in viscosity indicates reaction ofthe resin to form higher molecular weight products.

Two latent catalysts, phenyl hydrogen maleate and phenyltrifluoroacetate, were evaluated as well as two conventional acidcatalysts, diphenyl hydrogen phosphate and sulfuric acid. Each catalystwas added to a resin at a concentration of 2 weight percent, based onthe weight of the catalyst free resin. The phenyl hydrogen maleate wasprepared from maleic anhydride and excess phenol and was added as aphenol solution. The concentration of the phenyl hydrogen maleate wasabout 4 weight percent.

The polymer used was a phenol-formaldehyde resole resin havinghemiformal groups as disclosed in the above cited U.S. Application Ser.No. 340,853. It was made by charging into a 5 gallon vessel equippedwith a water separator 7058 grams of phenol, 35.3 grams of zinc acetatedihydrate and 494 grams of toluene. The solution was stirred and heatedto about 100° C., after which 9206 grams of an aqueous solution offormaldehyde containing 48.9 weight percent formaldehyde were meteredinto the reaction mixture over five hours. There was an initial mildexotherm, which was easily moderated by regulating the source of heat.Water added with the formaldehyde was then removed azeotropically withthe toluene using the water separator and a condenser. After all theaqueous formaldehyde solution had been added, the mixture wasazeotropically distilled for about 1.5 hours at atmospheric pressure toa temperature of about 110° C. The toluene and water were removed as anazeotropic vapor mixture which was condensed using the water separator.The heat-source was then removed and the distillation continued under areduced pressure of 50 mm Hg for about 1/2 hour. Total water recoveredcorresponded to about 106% of the water added in the aqueousformaldehyde solution. The resulting composition has a viscosity(Brookfield Model RVT) of about 4000 cp at 29° C.

Resins containing the catalysts and a catalyst-free resin, forcomparison, were evaluated for activity at cure temperatures by usingstandard differential calorimeter apparatus and the peak exothermdetermined. The calorimeter was a Dupont Differential ThermoanalyzerModel 990 equipped with a pressure cell. The data are summarized inTable I. As seen by the peak exotherms, shown in Table I, the curingactivity of the latent catalysts of the invention approaches theactivity of a conventional catalyst used in the art, diphenyl hydrogenphosphate.

Catalyst containing resins were evaluated for stability at storagetemperatures by allowing them to stand at a temperature of 25° C. to 30°C. and the time determined in which the viscosity doubled. These times(t₂) for each resin are shown in Table I. As shown by these times, thestability of the resins containing the latent catalysts used in theinvention are significantly greater than resins containing conventionalcatalysts. The decreased stability of the phenyl trifluoroacetatecontaining resin is probably due to its extremely facile hydrolysis withthe trace amounts of water in the resin. The decreased stability of thephenyl hydrogen maleate is probably due to the fact that phenyl hydrogenmaleate is itself a weak acid.

                  TABLE I                                                         ______________________________________                                                        Peak Evotherm                                                 Catalyst        Temperature (°C.)                                                                    t.sub.2 (days)                                  ______________________________________                                        None            219           250                                             Phenyl hydrogen maleate*                                                                      158           3                                               Phenyl trifluoroacetate*                                                                      133           1                                               Diphenyl hydrogen                                                             phosphate**     127           0.17                                            Sulfuric acid** --            <0.1                                            ______________________________________                                         *Latent catalyst                                                              **Conventional acid catalyst                                             

EXAMPLE 2

A thermosetting resin was prepared as in Example 1. Phenyl hydrogenmaleate was prepared by reacting excess phenol with maleic anhydride andwas added to the above resin.

The resin was cured using a liquid injection molding apparatus describedin U.S. Patent Application Ser. No. 139,906, filed Apr. 14, 1980 byAngell. Fiberglass mat was placed into a mold, the mold was preheated to150° C. and clamped with a hydraulic press. An exothermic reactionensued. When the mold cooled to its initial preheated temperature ofabout 150°, the cured composite plaques were removed from the mold. Thecycle times were measured and are shown in Table II.

The plaques were evaluated for their physical properties. In theseevaluations the following standard tests were used:

Flexural Modulus--ASTM D790

Flexural Strength--ASTM D790

Notched Izod (Impact)--ASTM D256

Tensilte Modulus--ASTM D638

Tensile Strength--ASTM D638

Elongation--ASTM D638

The glass fiber used was glass fiber mat, designated as type AKMavailable from PPG Industries, Pittsburgh, Pennsylvania.

In Table II are summarized the results of the tests, the glass contentexpressed as weight percent glass, based on the weight of the curedcomposite, mold cycle time in seconds, and the concentration of thecatalyst expressed as weight percent phenyl hydrogen maleate based onthe weight of the catalyst-free resin.

The data in Table II show the use of the latent catalysts of theinvention in curing resins to form composites. Excellent physicalproperties of the composites as well as short cycle times are shown.

                                      TABLE II                                    __________________________________________________________________________    Catalyst Mold Flexural                                                                             Flexural                                                                             Tensile                                                                              Tensile    Notched                         Conc.                                                                              Wt. %                                                                             Cycle                                                                              Modulus                                                                              Strength                                                                             Modulus                                                                              Strength                                                                             %   Izod                            (wt. %)                                                                            Glass                                                                             (sec)                                                                              (psi × 10.sup.-6)                                                              (psi × 10.sup.-3)                                                              (psi × 10.sup.-3)                                                              (psi × 10.sup.-3)                                                              Elong.                                                                            (ft-lbs/in)                     __________________________________________________________________________    9.8  51  150  1.69   33.3   1.45   23.0   2.2 26                              9.8  53  120  1.84   36.3   1.55   24     2.1 25                              9.8  52   90  1.45   29.5   1.47   22.8   2.0 28                              3.9  52  300-400                                                                            1.48   24.3   1.50   22.7   1.9 21                              3.9  60  300-400                                                                            2.2    35.7   1.67   25.6   2.1 35                              __________________________________________________________________________

We claim:
 1. In a process for the curing of a thermosetting resin whichis curable using an acid catalyst, the improvement being the use of aphenyl ester of a carboxylic acid as a latent catalyst, said phenylester being hydrolyzable at a temperature greater than 100° C. and inthe presence of water to form a carboxylic acid having a pK_(a) of about2 or less.
 2. A process as in claim 1 wherein the thermosetting resincomprises a hemiformal having any one of the following structures:##STR5## wherein R is a monovalent radical selected from the groupconsisting of alkyl from 1 to 18 carbon atoms, cycloalkyl from 5 to 8carbon atoms, aryl containing from 1 to 3 aromatic rings, aralkyl,alkaryl, alkoxy containing from 1 to 18 carbon atoms, aroxy containing 1to 3 aromatic nuclei, a halide radical, alkyl sulphido having from 1 to18 carbon atoms, aryl sulphido having from 1 to 3 aromatic nuclei, and aradical derived from linseed oil or tung oil, n is a positive number ofat least 1, b is 1 to about 5, c is 1 to about 3, and d is 0 to about 2,x is 0 to 3, the sum of c and d is at least 1 and no greater than 3 andthe sum of c, d, and x is at least 1 but no greater than 5, where x=0for at least 50 mole percent of the hemiformal, and with respect to theR substituent, at least 2 of the ortho- and para- positions are free inrelation to the --OH and --O(CH₂ O)_(n) H groups.
 3. A process as inclaim 1 wherein the thermosetting resin comprises a phenol-formaldehyderesin having the formula: ##STR6## wherein a is from 0 to 3, b is 0 to1, the sum of a and b does not exceed 3, the sum of c and d is from 2 toabout 20, the mole fraction d/(c+d) is 0.4 to 0.9, R is --(CH₂ O(CH₂O)_(e) H, e is 0 to about 5, and X is a monovalent radical selected fromthe group consisting of alkyl from 1 to 18 carbon atoms, cycloalkyl from5 to 8 carbon atoms, aryl containing from 1 to 3 aromatic rings,aralkyl, alkaryl, alkoxy containing from 1 to 18 carbon atoms, aroxycontaining 1 to 3 aromatic nuclei, a halide radical, alkyl sulphideshaving from 1 to 18 carbon atoms, aryl sulphides having from 1 to 3aromatic nuclei, wherein for at least one of the R or --O(CH₂ O)_(e) Hgroups, e is at least 1 and wherein at least 50 mole percent of the##STR7##
 4. A process as in claims 1, 2 or 3 wherein the latent catalystis an ester of a polymer having acid groups incorporated into itsstructure.
 5. A process as in claims 1, 2 or 3 wherein the latentcatalyst is phenyl hydrogen maleate or phenyl trifluoracetate.
 6. Aprocess as in claim 1, 2 or 3, a said process being a liquid injectionprocess wherein the thermosetting resin is injected directly into amold.
 7. A process as in claim 1, 2 or 3, a said process being a sheetmolding compounds process.
 8. A process according to claim 2 wherein nis from about 1 to about
 5. 9. A process according to claim 2 wherein nis from about 1.2 to about 2.5.
 10. A process according to claim 3wherein the mole fraction d/c+d is from 0.6 to 0.8.